This invention relates to dual surface coating methods in which a first liquid coating is applied to one surface of a continuously running support web and a second liquid coating is thereafter applied to the other or opposite surface of the web before the first applied coating is dried and solidified.
In the case of photo-sensitive photographic materials, magnetic recording materials, pressure-sensitive copying sheets or the like, it is often required to provide a liquid coating on both surfaces of a web. Heretofore a method has been employed in which a coating is applied to one surface of the web, and after the coating layer has dried and solidified a second coating is applied to the other surface of the web. This method of repeated drying is low in efficiency, however, and therefore a variety of other methods have been proposed to improve production.
One such method is disclosed in Japanese Patent Publication No. 29944/1974 and in Laid Open Japanese Patent Application No. 138036/1975, wherein coating liquids are applied to both surfaces of a web simultaneously by an extrusion method, and with the web transport being controlled by the coating liquid discharge pressure, coating layers are formed on both surfaces of the web. This method suffers from the disadvantage that the web position is liable to vary, however, and thus the distance or spacing between the web and the coating devices correspondingly varies and makes it difficult to uniformly coat the web with the liquids.
In another method when a coating layer coated on one surface of a web has set, the other surface of the web is coated with a liquid by bead coating, extrusion coating, or doctor coating techniques to form a second layer while the surface of the first coating layer is supported by gas pressure. This method also suffers from positional or "floating" variations of the web in a direction perpendicular to its surface, which makes it extremely difficult to maintain a constant distance between the coating device and the web and thus leads to uneven coatings. To overcome these difficulties it has been attempted to support the web by static pressure as disclosed in Japanese Patent Publication Nos. 17853/1974, (corresponding to U.S. Pat. No. 3,496,005) 44108/1974, 19130/1975 and 38737/1976. To obtain a uniform thickness of coating layer by the bead, extrusion or doctor coating methods, however, it is necessary to restrict spacing variations between the coating device and the web to the order of one micron. Accordingly, even using static support pressure techniques it is considerably difficult to precisely control the position of the web, because the maintenance of unvarying static pressure is extremely difficult. It is thus impossible to completely solve the problem of irregularities in the film thickness. When such irregularities are due to float variations of the web, as the frequency x the amplitude of the float vibrations increases, or the frequency x the amplitude/web speed increases, uneven films having lateral defects are formed, which is a fatal defect. The term "lateral defect" used in this specification means an uneven thickness of the coating layer in the longitudinal direction of the web.